Combustion gas desulfurization method

ABSTRACT

Process for desulfurizing combustion gas wherein the gas is fractionated into at least two flows which pass through distinct passageways (4 and 5) and receive particles of a solid desulfurizing agent coming from distinct injectors (7 and 8) . The two particle-charged flows meet (9) , are separated again (10 and 11) and meet again thereafter (12). The process can be repeated several times. The final mixture is carried off and fractionated into desulfurized gas and recovered solid particles.

BACKGROUND OF THE INVENTION

This invention relates to a process for desulfurizing combustion gas.

The strict regulations limiting in many countries and many regions theemissions of sulfur oxides through the gaseous effluents of heatgenerators forbid the use of fuels with a high sulfur content which,besides, may show definite economical advantages: it is the case of somecoals having certain similarities to lignites, and of petroleum residuesstemming from refining processes.

French patent 2,609,150 describes a process for reducing the emissionsof sulfur oxides and particularly the ones of sulfur dioxide coming froma heat generator accepting fuels with a high sulfur content. Accordingto this process, a powdered desulfurizing agent is injected into thecombustion gases and the resulting mixture is circulated in adesulfurization zone, then the at least partially desulfurated gases arepartly separated from the powdered desulfurizing agent which can berecycled or discharged.

A similar process is described in the French patent application EN.88/12,363. In this patent application, the mixing of the desulfurizingagent particles with the combustion gases is carried out in a turbulencezone located upstream from the desulfurization zone.

With this type of desulfurization, the rate of use of calcic ormagnesian absorbents such as calcium and magnesium oxides and carbonatesremains limited. The reason for this phenomenon has not been totallycleared up, but it seems that a skin which partly prevents the reagentsfrom getting into the grains, forms at the surface of these grains.

A process allowing to remedy the drawbacks mentioned above andparticularly allowing to obtain high desulfurization rates and toincrease the rate of use of the absorbent, at a lower cost, has beendiscovered.

SUMMARY OF THE INVENTION

More precisely, this invention is directed to a process fordesulfurizing combustion gases, wherein a grained (i.e. particulate)desulfurizing agent is injected into a flow or stream of combustiongases, has been discovered; said process comprises the following stages:

(a) fractionating the combustion gas into at least two streams,

(b) adding a particulate desulfurizing agent to each one of the at leasttwo streams,

(c) directing the resulting streams containing particulates of thedesulfurizing agent towards one another, in order to cause the meetingand the mixing thereof,

(d) carrying off the resulting mixture of gas and of solid particles,

(e) fractionating the discharged mixture from (d) into at least twostreams,

(f) again directing the resulting streams towards one another, in orderto cause the meeting and the mixing thereof, and

(g) separating the desulfurized gases from solid particles of thedesulfurizing agent.

After the separation, the particles can be recycled if they still show asufficient desulfurizing power, but this is not always necessary becauseof the improvement of the rate of use per pass obtained according to theinvention.

The grain or particle size of the particles usually ranges from 1 to 100micrometers, preferably from 2 to 25 micrometers, and the densitythereof ranges from 500 to 4,000 kg/m³.

The desulfurization temperature, that is the temperature obtained aftermixing the gases with the desulfurizing agent acting as an absorbent,can be selected between 700° and 1,100° C.

An important element is the rate of the flow of the suspension of thegas and of the particles. This rate advantageously ranges from 10 to 80m/s, preferably from 20 to 30 m/s. These rates lead to turbulence levelswhich are high enough to provide a homogeneous mixing of the absorbentin the fumes and thereby to obtain high rates of use of the absorbent.

The concentration of the particles in the gas can, for example, rangefrom 0.02 to 1 kg/Nm³, preferably from 0.05 to 0.2 kg/Nm³.

According to a preferred embodiment procedure, additional particles of adesulfurizing agent are injected in the flows of stage (f) and theresulting mixture of gas and particles is carried off following adirection substantially perpendicular to the one of said flows. Stage(g) is carried out thereafter.

It is clear that the carrying off of the gases at each stage of theprocess where such a discharge is planned, is performed following thenormal direction of flow of the combustion gases, that is from thecombustion zone towards the zone of discharge of the gases of theprocess.

After stage (f), the suspension of particles in the gas canadvantageously be let flow into a downstream zone, which is not inaccordance with the features mentioned above, for example in a zone witha circular or a square section that can, if need be, comprise elementsmodifying the direction of flow of the suspension, before carrying outthe separation treatment of stage (g) . The residence time in thisdownstream zone can for example range from 0.1 to 2 s, preferably from0.5 to 1 s.

The mixing of the particles with the gas (stage b) is preferably carriedout by injecting into the flows of combustion gas a preformed suspensionof solid particles in an auxiliary gas flow, which can itself be acombustion gas or a recycled gas.

In this case, the rate of injection of the preformed suspension can forexample range from 20 to 150 m/s, preferably from 50 to 100 m/s.

In case of a recycling of the particles, the particles can either berecycled as they are, or they can be subjected to a well-known treatmentfor separating undesirable elements such as calcium or magnesiumsulfate, or to a treatment for regenerating the absorbent, that is tosay which increases the specific surface of the partly sulfatedabsorbent.

Ammonia or urea can be introduced into the desulfurization chamber,preferably upstream, to complete if need be the denitrification of thecombustion effluents. Air can also be added so that the desulfurizationis performed in an oxygen atmosphere, if the combustion stage is notcarried out with excess air.

The absorbents which are the most frequently used as desulfurizingagents are oxides, hydroxides and carbonates of alkaline earth metals,for example limestones (CaCO3) , hydrated limes (Ca(OH)2) or dolomites(CaMg(CO3)2). Fine-grain products from the cement industry can also beutilized before they are baked.

They can be injected into a flow of gas such as air or steam orsuspended in a liquid such as water.

Venturi tubes or other devices favouring an additional turbulence can beused in the axis of injectors 7 and 8, as mentioned in French patentapplication EN 88/12,363.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and the advantages thereof will be clear from reading thedescription hereafter given by way of a non limitative example, withreference to the accompanying drawings in which:

FIG. 1 is a longitudinal section of a device according to the invention,and

FIGS. 2 and 3 are cross-sections of two embodiment variants of thedevice according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

This figure shows a device comprising an elongated combustion chamber(1) with a square or a rectangular section, preferably horizontal, whichcan burn any type of solid or liquid fuel containing sulfur andeventually nitrogen, injected in a pulverized form into the chamber (1)by a burner (2) which also receives an oxidizing gas such as air. Thisburner is advantageously arranged following the axis of the chamber andat the lower end thereof. A plurality of burners can also be usedinstead of a single burner.

The fuel can be for example a vacuum residue, a fluxed asphalt, a solidfuel, a composite fuel oil/coal fuel or a coal/water suspension.

The fuel is preferably burnt without excess air or with very littleexcess air, in order to reduce at the maximum the forming of nitrogenoxides.

During the combustion, a part of the energy released is advantageouslydrawn through exchange pipes such as (3) so that the temperature of thecombustion gaseous effluents ranges between 700° C. and 1,200° C. Theseeffluents leave the chamber (1) through two passageways (4 and 5)delimited by a plate (6) and are sent back towards one another at thelevel XX' after receiving solid particles coming from the injectors (7and 8). The connection of the injectors to the source of solid particlesis not shown.

When the two opposite flowing streams meet in zone (9), an intensestirring or mixing occurs, thereby favouring homogenous distribution ofthe particles in the total effluents.

The particle suspension leaves zone (9) in a direction substantiallyperpendicular to the direction of flow of one of the suspension streamsat the level XX' by passing through the passageway(s) provided in theplates or panels (16). This stream of gas-particle suspension is oncemore separated in two opposite streams (10 and 11) which run into oneanother again in zone (12). This can be repeated once, as shown on FIG.1, or more. Plates or panels (17) comprising passageways for thesuspension separate each level, and plates such a (18) allow to separatethe streams. The suspension of particles in the gas passes thereafterinto zone (13), where the desulfurization process continues, and then incontact with the pipes or the plates of a heat-recovery boiler (14). Thesuspension is discharged through line (15). The solid particles can beseparated from the gas thereafter, as shown on the figure of Frenchpatent application EN 88/12,363.

The recovered particles can be partly sent back into the injectors (7)and (8).

The injectors (7 and 8) and the deflecting plate (6) can be seen on FIG.2, which is a cross-section of FIG. 1 following XX'.

The boiler can also have a circular section. In this case (FIG. 3), aseries of injectors such as 7a, 7b, . . . can be arranged oppositeinjectors 8a, 8b, . . . . The plate can then be circular.

We claim:
 1. A process for desulfurizing combustion gas wherein solidparticles of a desulfurizing agent are injected into a combustion gasand wherein said desulfurizing agent comprises oxides, hydroxides orcarbonates of alkaline earth metals, said process comprising thefollowing stages:(a) fractionating a stream of the combustion gas intoat least two gas streams; (b) introducing solid particles of thedesulfurizing agent into each one of the at least two streams; (c)directing streams containing particles of the desulfurizing agentsuspended in the combustion gas resulting from stage (b) towards oneanother, in order to cause meeting of the streams and mixing thereof ina mixing zone; (d) discharging a resulting mixture of gas and ofparticles of the desulfurizing agent from said mixing zone to anotherzone; (e) fractionating the discharged mixture in said another zone intoat least two additional streams, each additional stream being formed bycombustion gas and particles of the desulfurizing agent; (f) directingthe streams resulting from stage (e) towards one another in order tocause the meeting of the streams and mixing thereof; and (g)fractionating the mixture resulting from stage (f) into a gas, at leastpartially desulfurized, and into sulfur-enriched particles of saiddesulfurizing agent; the size of the particles of the desulfurizingagent ranging from 1 to 100 micrometers, the rate of flow of the streamsmeeting in stage (c) ranging from 10 to 80 m/s and the combustion gasresulting from combustion of a sulfur-containing fuel.
 2. A processaccording to claim 1, wherein the size of the particles of thedesulfurizing agent ranges from 2 to 25 micrometers.
 3. A processaccording to claim 1, wherein the rate of flow of the streams when theymeet in stage (f) ranges from 10 to 80 m/s.
 4. A process as claimed inclaim 1, wherein stages (e) and (f) are repeated several times.
 5. Aprocess according to claim 1, wherein the particles of the desulfurizingagent are also added to the streams directed to one another in stage(f).
 6. A process according to claim 1, wherein the mixing of theparticles of the desulfurizing agent with each one of the at least twostreams in stage (b) is performed by injecting into each one of thestreams of combustion gas a preformed suspension of the particles ofsaid desulfurizing agent in an auxiliary gas flow, said suspension beinginjected at a rate of 50 to 100 m/s.
 7. A process according to claim 1,wherein the desulfurizing agent is selected from the group consisting oflimestone, hydrated lime and dolomite.
 8. A process according to claim1, wherein the temperature of the combustion gas ranges between 700° C.and 1200° C. at stage (b).